Nonvolatile storage element and method of manufacturing thereof

ABSTRACT

A method of manufacturing a variable resistance nonvolatile memory element includes: forming a lower electrode layer above a substrate; forming, on the lower electrode layer, a variable resistance layer including an oxygen-deficient transition metal oxide; forming an upper electrode layer on the variable resistance layer; and forming a patterned mask on the upper electrode layer and etching the upper electrode layer, the variable resistance layer, and the lower electrode layer using the patterned mask, wherein in the etching, at least the variable resistance layer is etched using an etching gas containing bromine.

TECHNICAL FIELD

The present invention relates to a variable resistance nonvolatilememory element and a method of manufacturing the same.

BACKGROUND ART

In recent years, variable resistance nonvolatile memory elements madeusing, as a storage material, a variable resistance material including atransition metal oxide which is oxygen-deficient compared to atransition metal oxide having stoichiometric composition have beenproposed. Such a nonvolatile memory element includes an upper electrodelayer, a lower electrode layer, and a variable resistance layer betweenthe upper electrode layer and the lower electrode layer. Resistance ofthe variable resistance layer reversibly changes upon application of anelectrical purse between the upper electrode layer and the lowerelectrode layer. Information can be stored in the nonvolatile memoryelement in a non-volatile manner by associating the information with thevalues of the resistance (see Patent Literature (PTL) 1 for example).Such variable resistance nonvolatile memory elements are expected to besmall and fast and consume a small amount of power compared to flashmemories having floating gates.

CITATION LIST [Patent Literature] [PTL 1]

-   Japanese Unexamined Patent Application Publication No. 2007-235139

SUMMARY OF INVENTION Technical Problem

However, there is a problem that the conventional variable resistancenonvolatile memory elements have characteristics which vary more widelythan an expectation based on thickness and composition of variableresistance layers and electrodes and dimensions and configurations ofphotoresist masks after lithography or configurations of variableresistance layers and electrodes after dry etching. This leads to aproblem that a larger-size nonvolatile memory device has poor retentionor inappropriately changes in resistance in bits having the poorestcharacteristics (tail bits) due to variation among nonvolatile memoryelements.

Conceived to address the problems, the present invention has an objectof providing variable resistance nonvolatile memory elements with lessvariation in characteristics thereamong and a method of manufacturingthe variable resistance nonvolatile memory elements.

Solution to Problem

In order to achieve the above-described object, provided is a method ofmanufacturing a nonvolatile memory element according to an embodiment ofthe present invention which includes: forming a lower electrode layerabove a substrate; forming, on the lower electrode layer, a variableresistance layer including an oxygen-deficient transition metal oxide;forming an upper electrode layer on the variable resistance layer; andforming a patterned mask on the upper electrode layer and etching theupper electrode layer, the variable resistance layer, and the lowerelectrode layer using the patterned mask, wherein in the etching, atleast the variable resistance layer is etched using an etching gascontaining bromine.

Advantageous Effects of Invention

The present invention provides variable resistance nonvolatile memoryelements with less variation in characteristics thereamong and a methodof manufacturing the nonvolatile memory elements.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a cross-sectional view illustrating a configuration of anonvolatile memory element according to an embodiment of the presentinvention.

FIG. 1B is an enlarged sectional view illustrating a configuration ofthe variable resistance element according to the embodiment of thepresent invention.

FIG. 2A is a cross-sectional view illustrating a step of a method ofmanufacturing the nonvolatile memory element according to the embodimentof the present invention.

FIG. 2B is a cross-sectional view illustrating a step of the method ofmanufacturing the nonvolatile memory element according to the embodimentof the present invention.

FIG. 2C is a cross-sectional view illustrating a step of the method ofmanufacturing the nonvolatile memory element according to the embodimentof the present invention.

FIG. 2D is a cross-sectional view illustrating a step of the method ofmanufacturing the nonvolatile memory element according to the embodimentof the present invention.

FIG. 2E is a cross-sectional view illustrating a step of the method ofmanufacturing the nonvolatile memory element according to the embodimentof the present invention.

FIG. 2F is a cross-sectional view illustrating a step of the method ofmanufacturing the nonvolatile memory element according to the embodimentof the present invention.

FIG. 2G is a cross-sectional view illustrating a step of the method ofmanufacturing the nonvolatile memory element according to the embodimentof the present invention.

FIG. 2H is a cross-sectional view illustrating a step of the method ofmanufacturing the nonvolatile memory element according to the embodimentof the present invention.

FIG. 3A illustrates advantageous effects of the nonvolatile memoryelement according to the embodiment of the present invention.

FIG. 3B illustrates advantageous effects of the nonvolatile memoryelement according to the embodiment of the present invention.

FIG. 4 shows conditions for analyses using XPS in FIG. 3A and FIG. 3B.

FIG. 5A illustrates advantageous effects of the nonvolatile memoryelement according to the embodiment of the present invention.

FIG. 5B illustrates advantageous effects of the nonvolatile memoryelement according to the embodiment of the present invention.

FIG. 6 illustrates advantageous effects of the nonvolatile memoryelement according to the embodiment of the present invention.

FIG. 7 illustrates a state of the nonvolatile memory element remainingin a high resistance state.

FIG. 8 shows concentration distributions of fluorine in a thin film ofTaO_(x) against depth measured using secondary ion mass spectrometry(SIMS) before and after dry-etching treatment using a mixed gas of C₅F₈,O₂ and Ar.

FIG. 9 shows concentration distributions of oxygen in a thin film ofTaO_(x) against depth measured using SIMS before and after dry-etchingtreatment using a mixed gas of C₅F₈, O₂, and Ar.

FIG. 10 shows concentration distributions of carbon in a thin film ofTaO_(x), against depth measured using SIMS before and after dry-etchingtreatment using a mixed gas of C₅F₈, O₂, and Ar.

FIG. 11 shows an example of resistance change characteristics of avariable resistance element made using the method according to thepresent embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

A method of manufacturing a nonvolatile memory element in an embodimentof the present invention includes: forming a lower electrode layer abovea substrate; forming, on the lower electrode layer, a variableresistance layer including an oxygen-deficient transition metal oxide;forming an upper electrode layer on the variable resistance layer; andforming a patterned mask on the upper electrode layer and etching theupper electrode layer, the variable resistance layer, and the lowerelectrode layer using the patterned mask, wherein in the etching, atleast the variable resistance layer is etched using an etching gascontaining bromine.

Here, for example, in the etching, the etching gas may contain hydrogenbromide.

By using this method, variable resistance nonvolatile memory elementswith less variation in characteristics thereamong are provided. Morespecifically, etched end faces of the variable resistance layer have aproduct of a bromine compound attached thereto formed as a result ofreaction with bromine. The bromine derives in decomposition in theetching plasma. The product of a bromine compound thus attached to theetched end faces reduces deoxidation of the etched end faces andimpurity implantation into the etched end faces due to etching gas. Thisleads to reduction of damage to the variable resistance layer during theetching. Variation in characteristics among nonvolatile memory elementsis thus reduced, so that the nonvolatile memory elements have initialresistance and operation characteristics with less variation thereamong,and thus prove to be quality nonvolatile memory elements with verylittle variation in characteristics.

Furthermore, hydrogen bromide, which is relatively a stable gas, isresponsive to oxides so poorly that etching using hydrogen bromideprogresses at a low rate. In this case, a variable resistance layer of ametal oxide is not etched. In other words, hydrogen bromide gas does notdamage the variable resistance layer but only contributes to protectionof etched end faces. Note that hydrogen bromide gas is commonly used ingeneral semiconductor processing.

It is therefore possible to manufacture the variable resistancenonvolatile memory elements using semiconductor processing including aconventional CMOS process. This means that the present invention can beadapted to semiconductor processing for finer design rules.

The inventors have conceived the present invention through elaboratestudies, in which the inventors found that variation in characteristicsfound in conventional manufacturing methods were due to damage duringetching.

More specifically, in the step of dry etching to form a variableresistance element in the conventional manufacturing method, etched endfaces of the target object are damaged by dry etching. Here, the damageduring etching means, for example, deoxidation of an oxide as a resultof reduction by dry etching using an etching gas. This leads to changein resistance of etched end faces of the oxide. The damage duringetching also means that implantation of an impurity, namely, fluorineinto etched end faces of an oxide during dry etching using a mixed gascontaining fluorine-containing gas as an etching gas. This also leads tochange in resistance of etched end faces of the oxide.

Thus, when a variable resistance layer including a metal oxide ispatterned by dry etching, etched end faces of the variable resistancelayer are damaged by deoxidation or impurity implantation during theetching, so that the resistance of the variable resistance layerchanges. When a variable resistance element includes a variableresistance region including such an etched end face damaged duringetching, the variable resistance element malfunctions due to deoxidationor impurity implantation.

This is the cause of malfunctions of conventional variable resistancenonvolatile memory elements, such as poor retention and inappropriatechange in resistance in tail bits when the nonvolatile memory elementshave larger capacity. The present invention has been conceived based onthe foregoing underlying knowledge.

Furthermore, to achieve the above-described object, in the forming ofthe variable resistance layer, in the method of manufacturing anonvolatile memory element in an aspect of the present invention, thevariable resistance layer is formed to include a transition metal oxidehaving resistance which is variable according to an oxygen amount in thetransition metal oxide, the resistance being increased by incorporationof an impurity in the variable resistance layer.

Here, for example, in the forming of the variable resistance layer, thevariable resistance layer may be formed to include a transition metaloxide having resistance which is increased by incorporation of fluorinein the variable resistance layer.

Furthermore, for example, in the etching, the etching gas may furtherinclude fluorine.

Furthermore, for example, in the etching, a bromine compound may beattached at least to an etched end face of the variable resistance layerwhile the variable resistance layer is being etched.

Furthermore, for example, the forming of the variable resistance layermay include: forming, on the lower electrode layer, a first variableresistance layer including a transition metal oxide; and forming, on thefirst variable resistance layer, a second variable resistance layerincluding a transition metal oxide having a degree of oxygen deficiencylower than a degree of oxygen deficiency of the first variableresistance layer.

By using this method, a conduction path (filament) is formed whichcauses change in resistance in the second variable resistance layer incontact with the upper electrode layer. When the etched end face of thevariable resistance layer is protected by a product of a brominecompound formed using a mixed gas containing a bromine compound as anetching gas, damage to the etched end face during etching such asdeoxidation and impurity implantation can be reduced. This reducesvariation in resistance among nonvolatile memory elements, and thenonvolatile memory elements stably operate even when a filament isformed near the etched end face.

Furthermore, for example, in the forming of the variable resistancelayer, the variable resistance layer may be formed to include atransition metal oxide having resistance which increases with a decreasein a degree of oxygen deficiency of the variable resistance layer, orthe variable resistance layer may be formed to include a metal oxidewhich is a tantalum oxide expressed as TaO_(x) where 0<x<2.5.

With this, the nonvolatile memory elements are capable of not onlyoperating fast but also allowing stable reversible rewriting to tailbits as well and have favorable retention characteristics.

Furthermore, for example, in the forming of the variable resistancelayer, the upper electrode layer may be formed to include one ofplatinum, iridium, and palladium.

In this configuration, variable resistance nonvolatile memory elementswith less variation in characteristics thereamong are provided.Furthermore, in order to achieve the above-described object, provided isa nonvolatile memory element according to an aspect of the presentinvention which includes: a lower electrode layer formed above asubstrate; a variable resistance layer formed on the lower electrodelayer and including an oxygen-deficient transition metal oxide; and anupper electrode layer formed on the variable resistance layer, whereinthe variable resistance layer has a side face with a bromine compoundattached thereto.

Furthermore, for example, the side face of the variable resistance layermay have a sidewall protection film thereon including the brominecompound.

Thus, according to the above-described embodiment, the variableresistance nonvolatile memory elements having less variation incharacteristics thereamong and a method of manufacturing the nonvolatilememory elements are provided.

For example, in the method of manufacturing a nonvolatile memory elementetched end faces of the variable resistance layer have a product of abromine compound attached thereto formed as a result of reaction withbromine. The bromine derives in decomposition in the etching plasma. Theproduct of a bromine compound thus attached to the etched end facesreduces deoxidation of the etched end faces and impurity implantationinto the etched end faces due to etching gas. This leads to reduction ofdamage to the variable resistance layer during the etching, so thatvariation in characteristics among nonvolatile memory elements can bereduced. Larger-capacity nonvolatile memory elements thus manufacturedhave initial resistance and operation characteristics with lessvariation thereamong, and prove to be quality nonvolatile memoryelements with favorable retention characteristics.

Note that such variable resistance nonvolatile memory element isapplicable to a large-scale semiconductor integrated circuit with all orpart of such functionality.

Hereinafter, an embodiment of the present invention is described withreference to drawings. Each of the exemplary embodiments described belowshows a general or specific example of the present invention. Thevalues, materials, constituent elements, layout and connection of theconstituent elements, steps, and the order of the steps in theembodiments are given not for limiting the present invention but merelyfor illustrative purposes only. Thus, among the constituent elements inthe following embodiments, a constituent element not included in theindependent claim reciting the most generic part of the concept of thepresent invention shall be described as a constituent element of apreferable embodiment.

A method of manufacturing a nonvolatile memory element according to anembodiment of the present invention is described below with reference tothe drawings. Note that description of elements denoted by the samereference signs may be omitted. Also note that the drawings showconstituent elements schematically for the sake of clarity, andtherefore the shapes thereof are not correct and the number of theconstituent elements is for simplicity of illustration.

Embodiment

FIG. 1A is a cross-sectional view illustrating a configuration of anonvolatile memory element according to an embodiment of the presentinvention. FIG. 1A shows an example of a single nonvolatile memoryelement 100. FIG. 1B is a cross-sectional view illustrating aconfiguration of the side faces of a variable resistance elementaccording to an embodiment of the present invention.

The nonvolatile memory element 100 shown in FIG. 1A includes: a variableresistance element 1, a substrate 11, a source-and-drain layer 12, agate 13, a first interlayer dielectric 14, a first contact 15, a secondcontact 16, a third contact 7, patterned wiring 18, and a secondinterlayer dielectric 19. The variable resistance element 1 includes: alower electrode layer 2, a variable resistance layer 3, and an upperelectrode layer 4. The variable resistance layer 3 includes a firstvariable resistance layer 31 and a second variable resistance layer 32.

The gate 13 is formed above the substrate 11 with a gate dielectricformed between the gate 13 and the substrate 11.

The source-and-drain layer 12 is formed in the substrate 11.

The first interlayer dielectric 14 is formed on the substrate 11 tocover the gate 13 and the source-and-drain layer 12. The firstinterlayer dielectric 14 may be a film of plasma TEOS or SiO₂.

The substrate 11, the gate 13, the gate dielectric, and thesource-and-drain layer 12 compose a transistor 20.

The first contact 15 is formed to penetrate through the first interlayerdielectric 14 to connect to one of the source and the drain in thesource-and-drain layer 12 and the lower electrode layer 2 of thevariable resistance element 1. The first contact 15 includes, forexample, tungsten or copper.

The variable resistance element 1 is formed on the first interlayerdielectric 14 and the first contact 15. More specifically, the lowerelectrode layer 2 is formed on the first contact 15 to connect to thefirst contact 15. The first variable resistance layer 31 is formed onthe lower electrode layer 2 and includes a first transition metal oxide.The second variable resistance layer 32 is formed on the first variableresistance layer 31 and includes a second transition metal oxide havinga degree of oxygen deficiency lower than the degree of oxygen deficiencyof the first oxide layer 31. The first variable resistance layer 31 andthe second variable resistance layer 32 thus form a layered structurewhich is the variable resistance layer 3 of the variable resistanceelement 1. The upper electrode layer 4 is formed on the second variableresistance layer 32. The first variable resistance layer 31 has athickness on the order of, for example, 20 nm to 100 nm inclusive. Thesecond variable resistance layer 32 has a thickness on the order of, forexample, 1 nm to 10 nm inclusive.

As shown in FIG. 1B, the variable resistance element 1 has sidewallprotection films 33 a on the side faces of the variable resistance layer3 (etched end face 33). The sidewall protection films 33 a are formed ofa bromine compound attached to the etched end face 33.

Here, the first variable resistance layer 31 is a first transition metaloxide layer including an oxygen-deficient transition metal oxide. Thesecond variable resistance layer 32 is a second transition metal oxidelayer including a transition metal oxide having a degree of oxygendeficiency lower than the degree of oxygen deficiency of the firsttransition metal oxide layer. In the present embodiment, for example, afirst transition metal included in the first transition metal oxidelayer and a second transition metal included in the second transitionmetal oxide layer are of the same transition metal. More specifically,the first variable resistance layer 31 is an oxygen-deficient firsttantalum oxide layer (TaO_(x)), and the second variable resistance layer32 is a second tantalum oxide layer (TaO_(y)). For TaO_(x) of the firsttantalum oxide layer, x satisfies 0<x<2.5, and for TaO_(y) of the secondtantalum oxide layer, y satisfies x<y. Furthermore, for TaO_(x) of thefirst tantalum oxide layer, x preferably satisfies 0.8≦x≦1.9, and forTaO_(y) of the second tantalum oxide layer, y preferably satisfies2.1≦y.

Note that the oxygen-deficient transition metal oxide is a transitionmetal oxide deficient in oxygen compared to stoichiometric composition.A nonvolatile memory element including a layered structure of TaO_(x)(0.8≦x≦1.9) and TaO_(y) (2.1≦y) will operate fast, so that thenonvolatile memory element characteristically allows stable reversiblerewriting. Also note that the degree of oxygen deficiency of atransition metal oxide refers to a rate of deficiency in oxygen tostoichiometric composition of the transition metal oxide. Generally, anoxide of stoichiometric composition has characteristics of an electricalinsulator, and an oxygen-deficient transition metal oxide hascharacteristics of an electrical conductor.

The second variable resistance layer 32 preferably has a degree ofoxygen deficiency lower than the degree of oxygen deficiency of thefirst variable resistance layer 31, and thus has a resistance higherthan the resistance of the first variable resistance layer 31. In thisconfiguration, voltage applied between the upper electrode layer 4 andthe lower electrode layer 2 to change resistance is distributed more tothe second variable resistance layer 32 than to the first variableresistance layer 31, so that oxidation-reduction reactions are likely tooccur more in the second variable resistance layer 32. Here, thematerial of the first transition metal included in the first variableresistance layer 31 and the material of the second transition metalincluded in the second variable resistance layer 32 may be either thesame or different. Examples of a transition metal to be included in thefirst metal oxide layer include tantalum (Ta), titanium (Ti), hafnium(Hf), zirconium (Zr), niobium (Nb), and tungsten (W). Since transitionmetals exhibit two or more oxidation states, the resistance state of atransition metal can be changed in oxidation-reduction reaction.Furthermore, when the material of the first transition metal and thematerial of the second transition metal are different, it is preferablethat the second transition metal have a standard electrode potentiallower than the standard electrode potential of the first transitionmetal. This is because resistance change is caused by change in highresistance of a minute filament formed in the second variable resistancelayer 32 due to oxidation-reduction reactions therein. A material havinga higher standard electrode potential has a characteristic that thematerial is less susceptible to oxidation. When oxidation-reductionreactions occurs more in the second transition metal than in the firsttransition metal, the nonvolatile memory element is expected to operatemore stably.

For example, when the first variable resistance layer 31 includes thefirst tantalum oxide layer TaO_(x) (0.8≦x≦1.9), the first variableresistance layer 312 preferably has a thickness of 45 nm. When thesecond variable resistance layer 32 includes the second tantalum oxidelayer TaO_(y) (2.1≦y), the second variable resistance layer 32preferably has a thickness of 5 nm.

Note that the functions and effects according to the present inventionare also present when tantalum oxide is used as a material of a variableresistance layer, and the present invention is not limited to theforegoing example in which tantalum oxide is used. For example, thelayered structure may include layers of oxides of hafnium (Hf) or layersof oxides of zirconium (Zr).

For example, assume that the layered structure includes layers ofhafnium oxides. For a first hafnium oxide expressed by compositionformula of HfO_(x), x preferably satisfies 0.9≦x≦1.6, and for a secondhafnium oxide expressed by a composition formula of HfO_(y), ypreferably satisfies 1.8<y. For example, assume that the layeredstructure includes layers of zirconium oxides. For a first zirconiumoxide expressed by composition formula of ZrO_(x), x preferablysatisfies 0.9≦x≦1.4, and for the second zirconium oxide expressed by acomposition formula of ZrO_(y), y preferably satisfies 1.9<y.

The upper electrode layer 4 preferably has a thickness of 50 nm. Theupper electrode layer 4 includes a simple metal or an alloy having astandard electrode potential higher than the standard electrodepotential of the transition metal included in the variable resistancelayer 3, and may have either a single-layered structure or amultiple-layered structure. Here, the metal having a standard electrodepotential higher than the standard electrode potential of the transitionmetal included in the variable resistance layer 3 is preferably a noblemetal such as platinum (Pt), iridium (Ir), or palladium (Pd).

When the material of the variable resistance layer 3 is anoxygen-deficient transition metal oxide, the material of the upperelectrode layer 4 is such that the upper electrode layer 4 has astandard electrode potential higher than the standard electrodepotential of the transition metal of the oxygen-deficient transitionmetal oxide and that the lower electrode layer 2 has a standardelectrode potential lower than the standard electrode potential of theupper electrode layer 4. Thus, at the interface between the electrodehaving a higher standard electrode potential (upper electrode layer 4)and the variable resistance layer 3, oxidation-reduction reactions occurpreferentially in the variable resistance layer 3 according to appliedvoltage, and a high-oxygen variable resistance layer and a low-oxygenvariable resistance layer are thereby formed. The nonvolatile memoryelement 100 thus obtained stably operates. Specifically, when theoxygen-deficient transition metal oxide is a tantalum oxide, theelectrode material having a higher standard electrode potential (forexample, Pt, Ir, or Pd) is used as a material of the electrode incontact with the second tantalum oxide layer having a lower degree ofoxygen deficiency, and the electrode material having a lower standardelectrode potential (for example, tantalum (Ta), tantalum nitride (TaN),or titanium (Ti)) is used as a material of the electrode in contact withthe first tantalum oxide layer having a higher degree of oxygendeficiency.

The second interlayer dielectric 19 is formed to cover the side faces ofthe lower electrode layer 2, the side faces of the first variableresistance layer 31, and the side faces of the second variableresistance layer 32 and the side faces and top face of the upperelectrode layer 4.

The second contact 16 is formed to penetrate through the secondinterlayer dielectric 19 to reach the upper electrode layer 4. The thirdcontact 7 is formed to penetrate through the second interlayerdielectric 19 and the first interlayer dielectric 14 to reach one of thesource and the drain in the source-and-drain layer 12. As with the firstcontact 15, the second contact 16 and the third contact 7 includes, forexample, tungsten or copper.

The patterned wiring 18 is formed on the top face of the secondinterlayer dielectric 19 and includes first patterned wiring 181 andsecond patterned wiring 182 connecting to the second contact 16 and thethird contact 7, respectively. More specifically, the second contact 16connects the upper electrode layer 4 of the variable resistance element1 to the first patterned wiring 181, and the third contact 7 connectsthe one of the source and the drain in the source-and-drain layer 12 tothe second patterned wiring 182. The patterned wiring 18 includes, forexample, copper or an aluminum alloy.

The nonvolatile memory element 100 is thus configured.

However, the nonvolatile memory element 100 shown in FIG. 1A maydeteriorate in resistance change characteristics as shown in FIG. 7.FIG. 7 shows that the variable resistance element remains in a highresistance state and fails to change to a low resistance state despitealternate applications of a pulse for high resistance writing and apulse for low resistance writing. The inventors investigated the causeof the deterioration in resistance change characteristics, andconsidered that this was caused by change in composition of the variableresistance layer due to radical fluorine, which is included in plasma ofan etching gas, incorporated into the variable resistance layer whilethe variable resistance layer is exposed to the etching gas containing agaseous fluorine compound in the process of dry etching. The same mayapply to oxygen-deficient transition metal oxides having resistancechange characteristics.

The inventors carried out the following experiment to examine effects ofan etching gas containing a gaseous fluorine compound on filmproperties.

First, the inventors prepared samples each of which is a substrate witha tantalum oxide (TaO_(x)) deposited thereon, and analyzed the surfaceof the tantalum oxide by secondary ion mass spectrometry (SIMS).

Next, the surface of the tantalum oxide was treated with dry etchingusing a mixed gas of C₅F₈, O₂, and Ar, and then analyzed using secondaryion mass spectrometry (SIMS). FIG. 8 shows concentration distributionsof fluorine in the thin film of TaO_(x) against depth measured usingsecondary ion mass spectrometry (SIMS) before and after the dry-etchingtreatment using the mixed gas of C₅F₈, O₂, and Ar.

The vertical axis indicates counts (cps) of fluorine ions, and thehorizontal axis indicates the depth (nm) in the TaO_(x) film from thesurface thereof. The data of the measurement before the dry etching isplotted as blank circles, and the data of the measurement after the dryetching is plotted as solid circles. This result shows that fluorine wasincorporated in the surface layer of the thin film of TaO_(x) as aresult of dry etching using fluorine-containing gas. The result alsoshows that judging from the half-value width, fluorine was incorporatedin the region above a depth of 5 nm to the surface of the TaO_(x) film.Use of other etching gases containing fluorine compounds, such as CF₄,CHF₃, and SF₆ yielded similar results. Fluorine ions were observed nearthe surface before the dry etching was performed as well. This isconsidered as fluorine incorporated in the surface layer of the TaO_(x)film before the dry etching for any reason.

FIG. 9 and FIG. 10 respectively show concentration distributions ofoxygen and carbon in the thin film of TaO_(x) against depth before andafter the dry-etching treatment using the mixed gas of C₅F₈, O₂, and Ar.These results show that oxygen and carbon were hardly incorporated inthe surface layer of the TaO_(x) film.

From the above results, it is derived that in order to avoidincorporation of fluorine near the surface of a variable resistancelayer, a manufacturing method in which the variable resistance layer isnot exposed to an etching gas containing a fluorine compound, astructure of a nonvolatile memory element such that the variableresistance layer therein is not exposed to an etching gas containing afluorine compound, or an additional treatment to restore the originalstate of the deteriorated variable resistance layer after exposure to anetching gas containing a fluorine compound is necessary.

Use of an etching gas containing BCl₃ and Cl₂ prevents incorporation offluorine into a variable resistance film. However, it has also beenobserved that use of the etching gas resulted in decrease of oxygen inthe variable resistance film along with decrease in initial resistanceand variation in resistance of the variable resistance film.

Conceived to address these problems, the present invention provides amethod of manufacturing a nonvolatile memory element in whichincorporation of fluorine into a variable resistance layer and decreasein oxygen in the variable resistance layer are prevented.

The method of manufacturing the nonvolatile memory element 100 accordingto the present invention will be described below.

FIGS. 2A to 2H are cross-sectional views illustrating steps of themethod of manufacturing the nonvolatile memory element 100 according tothe embodiment of the present invention. In practice, the substrate 11has a large number of nonvolatile memory elements formed thereon. Thesedrawings show a single nonvolatile memory element thereon for simplicityof illustration. In addition, each of the drawings shows an enlargedview of the nonvolatile memory element for easy understanding.

First, in the step shown in FIG. 2A, the source-and-drain layer 12 isformed in the substrate 11, and the gate 13 is formed above thesubstrate 11. Next, the first interlayer dielectric 14 which is a filmof plasma TEAS or SiO₂ is formed. Next, the first contact 15 is formedto penetrate the first interlayer dielectric 14 to connect to one of thesource and the drain in the source-and-drain layer 12.

Next, in the step shown in FIG. 2B, the lower electrode layer 2, thefirst variable resistance layer 31, the second variable resistance layer32, and the upper electrode layer 4 are formed in this order above thefirst interlayer dielectric 14 to cover the exposed top face of thefirst contact 15, and then the hard mask layer 5 is formed to cover thetop face of the upper electrode layer 4.

Note that hereinafter, the lower electrode layer 2, the variableresistance layer 3, the first variable resistance layer 31, the secondvariable resistance layer 32, the upper electrode layer 4, and the hardmask layer 5 may be either in a state where they are etched in a patternor in a state where they are each in the form of a film.

More specifically, the lower electrode layer 2 of TaN having a thicknessof 30 nm is formed on the first interlayer dielectric 14. Next, thefirst variable resistance layer 31 of an oxygen-deficient tantalum oxideTaO_(x) (here, x=1.56) having a thickness of 30 nm is formed on thelower electrode layer 2, and then the second variable resistance layer32 of TaO_(y) (here, y 2.48) having a thickness of 5 nm is formed highas on the first variable resistance layer 31. The tantalum oxide TaO_(y)has a degree of oxygen deficiency lower than the degree of oxygendeficiency of the TaO_(x). Next, the upper electrode layer 4 of iridium(Ir) having a thickness of 80 nm is formed on the second variableresistance layer 32, and then the hard mask layer 5 of TiAlN having athickness of 100 nm is formed on the top face of the upper electrodelayer 4.

Here, the first variable resistance layer 31 is deposited by reactivesputtering using metal tantalum as a sputtering target under an argonatmosphere containing oxygen. Subsequently, the second variableresistance layer 32 is formed on the first variable resistance layer 31by plasma-oxidizing the top face of the first variable resistance layer31 under an oxygen atmosphere, so that the second variable resistancelayer 32 has a degree of oxygen deficiency lower than the degree ofoxygen deficiency of the first variable resistance layer 31. Morespecifically, TaO_(x) is deposited to form the 35-nm thick firstvariable resistance layer 31, and then the top face of TaO_(x) depositedis oxidized by plasma-oxidizing under an oxygen atmosphere to form, onthe first variable resistance layer 31 of TaO_(x), the 5-nm thick secondvariable resistance layer 32 of TaO_(y) having a degree of oxygendeficiency lower than the degree of oxygen deficiency of TaO_(x). Themethod of oxidization is not limited to plasma-oxidizing and may beperformed by a treatment having an oxidation effect on a surface, suchas a heat treatment under an oxygen atmosphere. Alternatively, insteadof the oxidization, TaO_(y) may be deposited to form the 5-nm thicksecond variable resistance layer 32 by reactive sputtering after thedepositing of a 30-nm thick TaO_(x) layer. Reactive sputtering allowsfor adjustment of the degree of oxygen deficiency of a layer by changingthe oxygen level of an atmosphere for sputtering or using a transitionmetal oxide as a sputtering target TaO_(y) may be deposited using, as asputtering target, metal tantalum or a tantalum oxide (for example,Ta₂O₅).

Next, in the step shown in FIG. 2C, a pattern of photoresist mask 60 isformed by exposure and developing.

Next, in the step shown in FIG. 2D, a pattern of the hard mask layer 5including, for example, TiAlN is formed by dry etching.

Next, in the step shown in FIG. 2E, the upper electrode layer 4, thevariable resistance layer 3, and the lower electrode layer 2 are formedinto a desired pattern by dry etching using a mixed gas containingbromine, and then the hard mask layer 5 is removed. The variableresistance element 1 is thus formed to include: the upper electrodelayer 4, the variable resistance layer 3 including the second variableresistance layer 32 and the first variable resistance layer 31, and thelower electrode layer 2.

More specifically, first, the upper electrode layer 4 is etched by dryetching using a mixed gas containing, for example, Cl₂ and Ar and, as amask, the hard mask layer 5 including a 100-nm thick film of TiAlNformed in the step of FIG. 2D. Next, the variable resistance layer 3including a tantalum oxide is etched by dry etching using a mixed gascontaining SF₆ and HBr (hydrogen bromide). Next, the lower electrodelayer 2 including tantalum nitride is etched by dry etching using amixed gas containing Cl₂ and Ar. Next, the hard mask layer 5 is removedby etching, so that the variable resistance element 1 is provided.

Here, in the dry etching performed as described above, the etching gasused at least for the patterning of the variable resistance layer 3contains a bromine compound, which is preferably hydrogen bromide.Accordingly, the etched end faces 33 of the variable resistance layer 3each have the sidewall protection film 33 a including a product derivedfrom a bromine compound. The sidewall protection films 33 a are formedon the etched end faces 33 as a result of reaction thereof with brominecontained in the etching gas. The product derived from a brominecompound protects the variable resistance layer 3 against damage duringthe etching, such as deoxydation and impurity implantation caused byreaction with the etching gas.

The sidewall protection film 33 a thus formed prevents the etched endface 33 of the variable resistance layer 3 from being damaged during theetching, so that variable resistance element 1 thus formed has lessdeterioration and less variation in characteristics.

A mixed gas containing a bromine compound may also be used as an etchinggas in the process of dry etching to form a pattern of the upperelectrode layer 4. This positively prevents the etched end faces 33 ofthe variable resistance layer 3 from being damaged during etching whenthe etched end faces 33 are exposed in the case where the upperelectrode layer 4 is over-etched.

A mixed gas containing a bromine compound may also be used as an etchinggas in the process of dry etching to form a pattern of the lowerelectrode layer 2. This prevents the etched end face 33 of the variableresistance layer 3 from being damaged during dry etching the lowerelectrode layer 2.

Furthermore, the step in which the hard mask layer 5 is removed is anon-limiting example. The hard mask layer 5 may be left without beingremoved. In this case, the second contact 16 is to be formed topenetrate the hard mask layer 5 as well to connect to the upperelectrode layer 4. This step will be described later. When the hard masklayer 5 includes an electrically conductive material, the second contact16 may be formed not to penetrate but to connect to the hard mask layer5.

Next, in the step shown in FIG. 2F, the second interlayer dielectric 19is formed to cover the first interlayer dielectric 14, the upperelectrode layer 4, the variable resistance layer 3, and the lowerelectrode layer 2.

Next, in the step shown in FIG. 2G, a second contact opening 16A isformed at the position where the second contact 16 is to be formed inthe following step, to penetrate the second interlayer dielectric 19 toreach the upper electrode layer 4. The second contact 16 is to connectto the upper electrode layer 4 of the variable resistance element 1.

Next, in the step shown in FIG. 2H, the second contact 16 is formed inthe second contact opening 16A. Next, the third contact 7 is formed topenetrate the second interlayer dielectric 19 and the first interlayerdielectric 14 to connect to the other of the source and drain in thesource-and-drain layer 12. Next, on the top face of the secondinterlayer dielectric 19, the first patterned wiring 181 is formed toconnect the second contact 16 and the second patterned wiring 182 isformed to connect to the third contact 7.

The nonvolatile memory element 100 shown in FIG. 1 is thus made.

Nonvolatile memory elements with less variation in characteristics arethus made using the method in which the etching of the variableresistance layers of nonvolatile memory element 100 is performed usingthe above-described mixed gas containing bromine. The nonvolatile memoryelement 100 capable of stable operation may be implemented by providingthe variable resistance element 1 as, for example, a nonvolatile memoryelement including one transistor per nonvolatile memory cell.

The nonvolatile memory element 100 thus made has the followingadvantageous effects.

FIG. 3A and FIG. 3B illustrate advantageous effects of the nonvolatilememory element according to the embodiment of the present invention.

FIG. 3A and FIG. 3B show elemental composition of the TaO_(x) surface ofthe first variable resistance layer 31 analyzed by X-ray photoelectronspectroscopy (XPS). Each of FIG. 3A and FIG. 3B includes a result ofanalysis of the TaO_(x) surface etched using a mixed gas containing abromine compound used in the present embodiment, for example, a mixedgas of HBr and SF, at a flow ratio of 11 and results of analysis inComparison example 1 and Comparison example 2. In Comparison example 1,a TaO_(x) surface was etched using a mixed gas 1, and in Comparisonexample 2, a TaO_(x) surface was etched using a mixed gas 2. The mixedgas 1 and the mixed gas 2 included no bromine compound. FIG. 3A and FIG.3B each also include a result of analysis of the TaO_(x) surface of thefirst variable resistance layer 31 before being etched.

The mixed gas 1 includes gases of Ar, Cl₂, and CHF₃ at a flow ratio of20:10:1. The mixed gas 2 includes gases of BCl₃ and Cl₂ at a flow ratioof 1:1.

FIG. 3A shows the results of measurement of oxygen amounts in theTaO_(x) surface. FIG. 3B shows the results of measurement of impurityamounts (the amounts of fluorine, denoted as F amount in FIG. 3B) in theTaO_(x) surface. Here, the impurity amount (the amount of fluorine) isthe amount of fluorine among elements (impurities) other than tantalumand oxygen detected in XPS. In other words, the amount of an impurity(the amount of fluorine) means the amount of implanted fluorine. In FIG.3A, the larger the decrease in the oxygen amount from the initial stateis, the more the Tao_(x) film is deoxidized by being damaged duringetching. Similarly in FIG. 3B, the larger the amount of impurity (theamount of fluorine) is, the more the TaO_(x) film is damaged by impurityimplantation during etching. Note that XPS was performed under theconditions shown in FIG. 4.

FIG. 4 shows the conditions for the analysis using XPS shown in FIG. 3Aand FIG. 36. More specifically, XPS for the above-described analyses wasperformed under the conditions of a beam diameter of 100 m, an electronanalyzer angle of 45 degrees, a pass energy of 23.5 V, a referenceelement peak of C1s (285.2 eV), and the source X-ray of AlK (1486.6 eV).Furthermore, the X-ray irradiation was performed on a spot, at a stepsize of 0.100 eV, using irradiation energy in HP mode, at a vacuum of3.4 10⁻⁹ Torr. without using a neutralization gun. The analyses usingXPS was performed using Quantum 2000 made by ULVAC-PHI, Incorporated.

In the case of Comparison example 1, as shown in FIGS. 3A and 3B,decrease in the oxygen amount was reduced while the impurity amount (Famount) significantly increased. It is therefore possible that in dryetching using the mixed gas 1, the variable resistance layer was damagedby impurity implantation during the dry etching.

In the case of Comparison example 2, as shown in FIGS. 3A and 36, theimpurity amounts (F amount) were comparable while there was asignificant decrease in the oxygen amount from the initial state. It istherefore possible that in dry etching using the mixed gas 2, thevariable resistance layer was damaged by deoxidation during the dryetching.

In the case of the present embodiment where a mixed gas containing abromine compound (in particular, hydrogen bromide), as shown in FIGS. 3Aand 36, there were a slight decrease in the oxygen amount and a slightincrease in the impurity amount from the initial state. In comparisonwith Comparison example 2, the decrease in the oxygen amount was small.In comparison with Comparison example 1, the impurity amount was small.In other words, in the present embodiment, use of a mixed gas containinga bromine compound reduces decrease in the oxygen amount and increase inthe amount of impurity, so that damage to the end face of the variableresistance layer during etching can be reduced.

FIGS. 5A, 5 b, and 6 illustrate advantageous effects of the nonvolatilememory element according to the embodiment of the present invention.

FIGS. 5A and 5B show characteristics of the nonvolatile memory element100 made using the method in the present embodiment and characteristicsof the nonvolatile memory elements made using the methods inabove-described Comparison example 1 and Comparison example 2. FIG. 5Ashows the number of malfunctioning bits among 256 kbits. FIG. 5B showsretention characteristics of tail bits at 85C. The retentioncharacteristics (life prediction) were evaluated using the followingmethod. First, for example, at each of the temperatures of 180 C, 150 C,and 125 C, a length of time is calculated which is taken by the currentvalue of the nonvolatile memory element in a low-resistance state toreach a reference value. Similarly, a length of time is calculated whichis taken by the current value of the nonvolatile memory element in ahigh-resistance state to reach a reference value. The reference valueis, for example, 50% of an initial current value. Next, activationenergy is calculated from Arrhenius plot to determine a time taken toreach the reference value at a temperature of 85 C. The life timeprediction (retention characteristics) is determined using thecalculation results.

As shown in FIG. 5A, in Comparison example 1, the number ofmalfunctioning bits among 256 kbits was 103 bits, and in Comparisonexample 2, the number of malfunctioning bits among 256 kbits was 193bits. In comparison, the number of malfunctioning bits among 256 kbitswas 0 bits in the present embodiment, which shows that the nonvolatilememory element even having increased capacity included no malfunctioningbit.

Furthermore, as shown in FIG. 5B, the retention at 85 C is approximately400 hours in Comparison example 1 and approximately 1.5 years (13000hours) in Comparison example 2. In comparison, the retention at 85 C inthe present embodiment is approximately 17 years (150000 hours). Thisshows that the present invention has an advantageous effect of improvingthe characteristics significantly.

In this manner, use of the mixed gas containing a bromine compound asused in the present embodiment decreases damage to an etched end faceduring etching even when a filament is provided close to the etched endface. This reduces possibility of occurrence of malfunctioning bits inthe nonvolatile memory element 100 according to the present embodiment,and thus has an advantageous effect of improving the characteristics oftail bits.

Next, FIG. 6 shows element content measured in analysis of the TaO_(x)surface of the variable resistance element 1 using XPS. The curvestherein show a result of Ir4f spectral analysis of the TaO_(x) surfaceetched using the mixed gas containing a bromine compound in the presentembodiment and a result of Ir4f spectral analysis of the TaO_(x) surfaceetched using the mixed gas not containing a bromine compound inComparison example 2. The analysis using XPS was performed under theconditions of incident energy of 150 eV, a pass energy of 100 eV, anenergy step of 0.1 eV, an acquisition time of 0.2 ms/step, and acumulated number of 25. The analyzer used is VG Scienta R4000WAL.

In the present embodiment, a product of a bromine compound (IrBr_(x))was detected in the TaO_(x) surface of the variable resistance element 1at 62 eV and 65 eV. In Comparison example 2, no product of a brominecompound was detected in the TaO_(x) surface of the variable resistanceelement.

This shows that in etching of the variable resistance element 1, use ofthe mixed gas in the present embodiment forms a product of a brominecompound (the sidewall protection films 33 a) on the etched end faces33, and thereby reduces damage caused by deoxidation and impurityimplantation during etching.

FIG. 11 shows an example of resistance change characteristics of avariable resistance element made using the method in the presentembodiment. FIG. 11 shows that stable resistance change characteristicswere obtained.

Thus, etching the variable resistance layer 3 of the variable resistanceelement 1 using the mixed gas in the present embodiment providesvariable resistance nonvolatile memory elements having less variation incharacteristics thereamong and a method of manufacturing the nonvolatilememory elements.

In the method of manufacturing a nonvolatile memory element in thepresent invention, a product of a bromine compound attaches to an etchedend face of a variable resistance layer to form a sidewall protectionfilm, so that deoxidation and impurity implantation due to etching gasare reduced. This leads to reduction of damage to the variableresistance layer in the etching. Variation in characteristics amongnonvolatile memory elements is thus reduced, so that nonvolatile memoryelements with larger capacity has initial resistance and operationcharacteristics free from variation therein and thus have favorableretention characteristics.

Bromine (Br) is likely to combine with an object to be etched to Form areaction product and attach to sidewalls. Thus, the product of a brominecompound forms the sidewall protection films 33 a as described above, sothat impurity implantation and deoxidation are prevented. Materials suchas TaO_(x) have varied resistance depending on the amount of oxygencontained therein. This variation in resistance is caused byimplantation of an impurity (for example, fluorine) which hindersmovement of oxygen ions included in the materials such as TaO_(x).Furthermore, decrease in the amount of oxygen contained in the variableresistance layer also causes decrease in resistance, that is, leak. Thesidewall protection films 33 a have an advantageous effect of preventingsuch variation in resistance and leak.

Thus, the nonvolatile memory element 100 capable of stable operation maybe implemented by providing the variable resistance element 1 in thepresent invention as, for example, a nonvolatile memory elementincluding one transistor per nonvolatile memory cell.

The present invention is not limited to the above-described embodimentsused as a basis of the description of the nonvolatile memory element andthe method of manufacturing the nonvolatile memory element according tothe present invention. A variable resistance element having a layeredstructure of transition metal oxides having different degrees of oxygendeficiency as shown in the present embodiment is provided with aprotective layer at least on the sidewall of a variable resistance layerso that implantation of an impurity (for example, fluorine) anddeoxidation can be prevented. Note that the implantation of an impurityhinders movement of oxygen ions included in the variable resistancelayer and thereby causes change in resistance thereof, and that thedeoxidation causes decrease in the amount of oxygen contained in thevariable resistance layer and thereby causes decrease in resistance,that is, leak. Damage to the variable resistance layer during etching isthus reduced and thereby variation in characteristics among nonvolatilememory elements is reduced, so that nonvolatile memory elements evenhaving a larger capacity are free from variation in initial resistanceand operation characteristics and have favorable retentioncharacteristics.

In other words, unless they depart from the spirit and scope of thepresent invention, variations of the embodiment which would occur tothose skilled in the art and embodiments in which the constituentelements in the present embodiment or the variations thereof, are alsowithin the scope of the present invention.

For example, the above-described case where the variable resistancelayer is etched using a mixed gas containing hydrogen bromide is anon-limiting example. The mixed gas may contain a material other thanhydrogen bromide. For example, use of a mixed gas containing bromineinstead of hydrogen bromide is expected to produce the same effect asthe present embodiment, and thus is within the scope of the presentinvention.

Alternatively, use of a mixed gas containing an element other thanhydrogen bromide is within the scope of the present invention as far asit satisfies the following conditions and produces the same effect asthe present embodiment. Specifically, use of a mixed gas is within thescope of the present invention when the mixed gas contains an elementsatisfying the following conditions so as to and produce the sameeffects: (1) the element has an atomic radius than fluorine, which is animpurity to the oxide; (2) the element is not to be substituted byoxygen; and (3) the element is irresponsive to oxygen.

INDUSTRIAL APPLICABILITY

The present invention is applicable to nonvolatile memory elements andmethods of manufacturing the nonvolatile memory elements, andparticularly to nonvolatile memory elements to be used in electronicdevices such as electronic digital appliances, memory cards, personalcomputers, and mobile computers, and method of manufacturing thenonvolatile memory elements.

REFERENCE SIGNS LIST

-   1 variable resistance element-   2 lower electrode layer-   3 variable resistance layer-   4 upper electrode layer-   5 hard mask layer-   7 third contact-   11 substrate-   12 source-and-drain layer-   13 gate-   14 first interlayer dielectric-   15 first contact-   16 second contact-   16A second contact opening-   18 patterned wiring-   19 second interlayer dielectric-   31 first variable resistance layer-   32 second variable resistance layer-   33 etched end face-   33 a sidewall protection film-   60 photoresist mask-   100 nonvolatile memory element-   181 first patterned wiring-   182 second patterned wiring

1. A method of manufacturing a nonvolatile memory element, the methodcomprising: forming a lower electrode layer above a substrate; forming,on the lower electrode layer, a variable resistance layer including anoxygen-deficient transition metal oxide; forming an upper electrodelayer on the variable resistance layer; and forming a patterned mask onthe upper electrode layer and etching the upper electrode layer, thevariable resistance layer, and the lower electrode layer using thepatterned mask, wherein in the etching, at least the variable resistancelayer and the lower electrode layer are etched using an etching gascontaining bromine.
 2. The method of manufacturing a nonvolatile memoryelement according to claim 1, wherein in the etching, the etching gascontains hydrogen bromide.
 3. The method of manufacturing a nonvolatilememory element according to claim 1, wherein in the forming of thevariable resistance layer, the variable resistance layer is formed tocomprise a transition metal oxide having resistance which is variableaccording to an oxygen amount in the transition metal oxide, theresistance being increased by incorporation of an impurity in thevariable resistance layer.
 4. The method of manufacturing a nonvolatilememory element according to claim 3, wherein in the forming of thevariable resistance layer, the variable resistance layer is formed tocomprise a transition metal oxide having resistance which is increasedby incorporation of fluorine in the variable resistance layer.
 5. Themethod of manufacturing a nonvolatile memory element according to claim1, wherein in the etching, the etching gas further comprises fluorine.6. The method of manufacturing a nonvolatile memory element according toclaim 1, wherein in the etching, a bromine compound is attached at leastto an etched end face of the variable resistance layer while thevariable resistance layer is being etched.
 7. The method ofmanufacturing a nonvolatile memory element according to claim 1, whereinthe forming of the variable resistance layer includes: forming, on thelower electrode layer, a first variable resistance layer comprising atransition metal oxide; and forming, on the first variable resistancelayer, a second variable resistance layer comprising a transition metaloxide having a degree of oxygen deficiency lower than a degree of oxygendeficiency of the first variable resistance layer.
 8. The method ofmanufacturing a nonvolatile memory element according to claim 1, whereinin the forming of the variable resistance layer, the variable resistancelayer is formed to comprise a transition metal oxide having resistancewhich increases with a decrease in a degree of oxygen deficiency of thevariable resistance layer.
 9. The method of manufacturing a nonvolatilememory element according to claim 1, wherein in the forming of thevariable resistance layer, the variable resistance layer is formed tocomprise a metal oxide which is a tantalum oxide expressed as TaO_(x)where 0<x<2.5.
 10. The method of manufacturing a nonvolatile memoryelement according to claim 1, wherein in the forming of the upperelectrode layer, the upper electrode layer is formed to comprise one ofplatinum, iridium, and palladium. 11-12. (canceled)
 13. The method ofmanufacturing a nonvolatile memory element according to claim 1, whereinin the etching, the upper electrode layer, the variable resistancelayer, and the lower electrode layer are etched using the etching gascontaining bromine.